1. Field of the Invention
This invention concerns a semiconductor device which includes an insulating substrate, having a predetermined wiring pattern thereon, adhered to a conductive substrate.
2. Description of the Prior Art
Conventionally, a semiconductor device including more than two semiconductor chips (called multichip semiconductor device) is utilized to meet the demand for a large scale integrated system.
FIG. 1 is a cross sectional view of a conventional multichip semiconductor device and FIG. 2 is a plan view thereof. FIG. 1 corresponds to the cross sectional view along the line I--I in FIG. 2. The device includes a conductive island portion 1, an insulating substrate 2 having a predetermined wiring pattern 3 thereon, two semiconductor chips 4, outer leads 5, bonding wires 6, adhesive 7 and 9 and mold resin 8. To electrically fix the semiconductor chips 4 to the wiring pattern 3 on the insulating substrate 2, a conductive adhesive, e.g., a paste containing silver (Ag) powder, is used as the adhesive 9. To simplify the production process, the conductive adhesive is also used as the adhesive 7 to fix the insulating substrate 2 to the conductive island portion 1.
Conventionally, the predetermined wiring pattern 3 on the insulating substrate 2 is composed of a copper leaf plated with gold (Au), for example. The Au plate is preferable to keep a good adhesion between the bonding wires 6 and the wiring pattern 3.
Usually, an elcectroplating method is used for the plating of the gold to insure a sufficient thickness of the Au plate layer. In the electroplating, a predetermined voltage supply is applied to the copper leaf. Thus, it is required that the copper leaf be continuously formed during the plating to assure sufficient gold plate thickness. Then, at the subsequent cutting process of the insulating substrate into the predetermined shape, the copper leaf plated with Au is simultaneously separated and a predetermined wiring pattern 3 is formed on the insulating substrate 2. Consequently, the wiring pattern 3 ends at the periphery of the insulating substrate 2 as shown in FIG. 2. If the amount of the conductive adhesive 7 and 9 is excessive, the adhesive may protrude outside and cause a short circuit between the conductive adhesive 7 and 9 and the wiring pattern 3. In addition, the conductive adhesive 7 may climb up the outside edges of the device and cause a short circuit with wiring layer 3.
Furthermore, the number of short circuits created is proportionally increased according to the number of semiconductor chips fixed on the substrate by the conductive adhesive, since the chance of contact between the conductive adhesive 9 under the semiconductor chip 4 and the adjacent wiring pattern 3 is increased. Thus, this makes it difficult to increase the number of the semiconductor chips mounted on each multichip device.